diff --git a/requirements/constraints.txt b/requirements/constraints.txt
index f50ae5d..b398986 100644
--- a/requirements/constraints.txt
+++ b/requirements/constraints.txt
@@ -12,4 +12,8 @@
 -c common_constraints.txt
  
 # incremental upgrade plan.
-django-simple-history<=3.1.1
\ No newline at end of file
+django-simple-history<=3.1.1
+
+
+# it is not availablein python3.9
+backports.zoneinfo;python_version<"3.9"